a) Find the product (1/2x2+y2) x-y

b) Simplyfy 5/3a2b x (-3/4ab3) x (-4/5a2b2) and find the value for 2= -2 and b= 1.
c) Divide x (5x2 - 80 by 5x (x+4).

Thanks in anticipation.

c) I will do the last one,

I assume you meant:
x(5x^2 - 80)/(5x(x+4))
= 5x(x^2 - 16)/(5x(x-4))
= (x-4)(x+4)/(x-4)
= x+4 , x ≠0, 4

without proper bracketing, I cannot make out the others. Use ^ to show exponents